1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method and, in particular, to an image forming apparatus and an image forming method that form a color image by using a rotatable developing device.
2. Description of the Related Art
Generally, image forming apparatuses that form a color image use four colors of toner which consists of three colored toner (cyan, magenta, and yellow) and black toner.
There are various methods for forming a color image by using such four colors of toner. In order to keep size of an image forming apparatus small, a structure is widely used in which four toner developer units or three colored toner developer units are evenly spaced on a rotatable developing device.
For a structure in which three colored toner developer units are arranged on a rotatable developing device, it is necessary to arrange a developer unit for black toner (black developer unit) independently of the rotatable developing device for colored toner. In business offices, the frequency of use of black-and-white images is still predominantly high in most cases. Therefore, in view of supplying black toner, such a structure, in which the black developer unit and the rotatable developing device for colored toner are disposed independently of each other, is advantageous in many respects.
Japanese Unexamined Patent Application Publication Nos. 5-241420 and 6-019271 disclose a technique regarding an image forming apparatus in which a black developer unit and a rotatable developing device for colored toner are disposed independently of each other.
A typical image forming apparatus forms a color image by sequentially superposing images corresponding to four colors of toner on a transfer belt or a transfer drum. Therefore, image formation for a color image requires a longer time than that for a black-and-white image.
For sequential development of each colored toner image using a rotatable developing device, it is necessary to rotate the rotatable developing device every time color is switched. In the case where three colored toner developer units are disposed on the rotatable developing device, since these developer units are spaced 120 degrees apart, the rotatable developing device is required to be rotated 120 degrees every time color is switched among cyan, magenta, and yellow. The time required for this rotation movement is one of the causes of increased time required for color image formation.
The patent documents mentioned above both disclose a technique that aims to reduce the time required for color image formation. For example, Japanese Unexamined Patent Application Publication No. 5-241420 discloses a technique that reduces the time required for color image formation by employing the unnecessity of waiting time for switching when operation shifts from a fixed black developing device to a rotatable developing device and vice versa.
Japanese Unexamined Patent Application Publication No. 6-019271 discloses a technique that reduces the time required for color image formation by using a structure in which an image for one color among the three colors is developed simultaneously with development of an image for black.
For an image forming apparatus in which colored toner developer units are arranged on a rotatable developing device, one of the most effective approaches for reducing the time required for color image formation is to increase the rotation speed of the rotatable developing device.
Rotating the rotatable developing device and thus moving the positions of the three colored toner developer units which are spaced 120° apart in a short time allows delay time associated with color switching to be reduced, resulting in a reduction in the time required for color image formation.
In order to rotate the rotatable developing device in a short time, it is necessary to increase angular acceleration of a driving motor that drives the rotatable developing device.
However, in general, increasing the angular acceleration of the driving motor increases the current consumption and power consumption of the driving motor. Additionally, it increases vibration and noise associated with driving.